SELECT 
  product_id, 
  feature_id, 
  variant_id 
FROM 
  cscart_product_features_values 
WHERE 
  product_id IN (
    13951, 13952, 13953, 13954, 13955, 13956, 
    13957, 13958, 13959, 13960, 13961, 
    13962
  ) 
  AND feature_id IN (618) 
  AND lang_code = 'en'

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost": 0.00340614,
    "nested_loop": [
      {
        "table": {
          "table_name": "cscart_product_features_values",
          "access_type": "range",
          "possible_keys": [
            "PRIMARY",
            "fl",
            "lang_code",
            "product_id",
            "fpl",
            "idx_product_feature_variant_id"
          ],
          "key": "lang_code",
          "key_length": "12",
          "used_key_parts": ["lang_code", "feature_id", "product_id"],
          "loops": 1,
          "rows": 12,
          "cost": 0.00340614,
          "filtered": 100,
          "attached_condition": "cscart_product_features_values.feature_id = 618 and cscart_product_features_values.product_id in (13951,13952,13953,13954,13955,13956,13957,13958,13959,13960,13961,13962) and cscart_product_features_values.lang_code = 'en'",
          "using_index": true
        }
      }
    ]
  }
}